Isopropylation of naphthalene by silica–alumina catalysts: coordination of active species determines the selectivity

Abstract

Tailoring the local coordination environment of acid sites in amorphous silica-alumina (ASA) catalysts remains a critical challenge for controlling reactivity and product selectivity in the isopropylation of naphthalene. Here, we demonstrate that a systematic co-precipitation strategy, specifically controlling the precursor-mixing phase and the pH-adjustment sequence, profoundly dictates the specific surface area and macroscopic acid-site distribution of ASA catalysts. By correlating these macroscopic properties with atomic-scale insights from advanced 2D 27Al Double Quantum-Single Quantum (DQ-SQ) MAS NMR, we reveal that direct mixing of Si and Al precursors at pH 8.7 selectively creates proximate Al(IV)–Al(V) pairs. We identify that these proximate pairs are closely associated with the strong acidity and the resulting performance enhancement. However, this strongly acidic environment promotes extensive over-alkylation into tri- and tetra-isopropylnaphthalene (PIPN). Conversely, when mixing is performed and the pH is subsequently adjusted to 9.7, it yields an ASA catalyst with a unique structural motif characterized by two proximal, distinct Al(VI) centers. This catalyst features a higher density of weak acid sites, which effectively suppresses over-alkylation to favor di-isopropylnaphthalene. These findings provide a clear link between macroscopic synthesis conditions and the atomic-level structural motifs of acid sites, enabling the rational design of ASA catalysts for targeted aromatic alkylation. © 2026 The Authors